1. Field of the Invention
This invention relates to a fluorescent lamp which does not exhibit objectionable interference colors and which has two different light-transparent layers adjacent the inner glass envelope surface. More particularly, this invention relates to fluorescent lamps having two non-luminescent, light-transparent layers adjacent each other and disposed on the inner surface of the glass envelope, each layer having a different index of refraction with one layer being particulate and having a particle size distribution such that the lamp does not exhibit optical interference colors from said layers.
2. Background of the Disclosure
It is well known in the fluorescent lamp industry that the starting voltage requirement of a fluorescent lamp is influenced by the surface resistance of the inner wall of the lamp envelope or tube. By using a conductive coating disposed adjacent the inner wall surface, the voltage necessary for ignition or starting the arc of a fluorescent lamp is substantially reduced. Most often the conductive coating is tin oxide doped with minor amounts of antimony or fluorine to make the oxide layer electrically conducting, since tin oxide of itself is a semiconductor. Indium oxide has also been used as a conductive coating. However, the use of a conductive coating creates its own problems in that it is somewhat subject to degradation by the mercury arc which cause it to discolor and turn grey during the life of the lamp, resulting in reduced light output. Consequently, it has become common to provide a protective layer on the conductive layer in order to overcome these disadvantages. The protective layer must be continuous, electrically non-conducting, and chemically inert in that it doesn't react with the arc, the phosphor or the mercury and it must also be substantially transparent to light radiation in the visible range. A layer of submicron particle size alumina such as Alon C or Degussa C, finely powdered aluminum oxide having a particle size range of 70-100 nanometers, has been commercially used as a protective layer to overcome the foregoing disadvantages. Other oxides which may and which have been used include metal oxides such as silica, yttria and zirconia. Such protective coatings are generally employed at a thickness within the range of about 500-800 .ANG. and 8,000-10,000 .ANG., at which thickness they are substantially transparent to the visible light radiation emitted by the lamp. The index of refraction of the conductive layer and that of the protective layer is different since they are different materials which makes the two layer combination act as an optical interference filter producing a visible coloration when the lamp is in an unlit condition. Slight variations in layer thickness can produce a striated, pearlescent effect which some find objectionable. This phenomena is more observable when the lamp is in the unlit condition than when it is in the lit condition. Although this phenomena has always existed, it has not been too objectionable with fluorescent lamps wherein the protective coating is in the range of either 500-800 .ANG. or greater than 6,000 .ANG..
Lamp manufacturers have recently started manufacturing more compact fluorescent lamps wherein the diameter of the lamp envelope has been substantially reduced for the same light output. This brings the conductive layer closer to the arc discharge. As a consequence, the thickness of the protective layer has been increased to the range of from about 2000-4000 .ANG. which has exacerbated and intensified the optical interference filter coloration effect to the point where some customers will not purchase such lamps. Consequently, there has been a need for fluorescent lamps which do not exhibit this objectionable coloration.